This invention relates to ambient temperature curable compositions, comprising an epoxy resin and an amine-functional curing agent and useful as coatings, adhesives or sealants, and to a process for curing the compositions.
The use of heterocyclic secondary amines such as piperidine, pyrrolidine, aminoethylpiperazine or anabasine as curing agents is discussed in the textbook xe2x80x9cHandbook of Epoxy Resinsxe2x80x9d by Lee and Neville published by McGraw-Hill, 1967. They are generally used in small amounts to promote mainly self-condensation of epoxide groups. Piperidine, for example, is customarily used at 5-7% by weight based on an epoxy resin such as the diglycidyl ether of bisphenol A (DGEBA). 2,3-Bipiperidine is described by Forostyan et al. in Plasticheskie Massy, 1965(1), 16-17 as providing cures with DGEBA in 2 hours at 80xc2x0 C.
US-A-4581454 describes adducts of aminohydrocarbyl piperazines and urea which are used as curing agents for epoxy resins, particularly in reaction injection moulding.
According to one aspect of the present invention, a process for forming a layer of cured epoxy resin on a substrate comprises coating the substrate with a composition comprising an epoxy resin and an amine-functional curing agent and allowing the coating thus applied to cure at ambient temperature, and it is characterised in that the curing agent comprises a material containing at least two heterocyclic secondary amine groups. By a heterocyclic secondary amine group we mean a secondary amine group in which the amine nitrogen atom forms part of a heterocyclic ring.
The invention also provides for the use of a compound containing at least two heterocyclic secondary amine groups as curing agent for an epoxy resin, characterised in that the epoxy resin and the compound containing at least two heterocyclic secondary amine groups are applied together as a coating to a substrate and allowed to cure on the substrate at ambient temperature.
The ambient temperature at which the coating is cured is generally below 40xc2x0 C. and frequently below 25xc2x0 C., and it may be below 10xc2x0 C. or even below 0xc2x0 C., down to xe2x88x9220xc2x0 C. for example. At these low temperatures the epoxy resin compositions of the invention cure more rapidly than known epoxy resin compositions such as those using a diprimary amine as curing agent. We believe that initial curing of the epoxy resin is essentially through epoxide/amine reaction rather than self-condensation of epoxide groups.
The epoxy resin of the composition can in general be any of the epoxy resins described in the above textbook by Lee and Neville, preferably a glycidyl-type epoxy resin containing glycidyl ether or ester groups. The epoxy resin can for example be a glycidyl ether of a bisphenol such as DGEBA or can be a condensed or extended glycidyl ether of a bisphenol. Such glycidyl ethers derived from a bisphenol generally have an epoxy functionality of 2 or slightly less, for example 1.5 to 2. The epoxy resin can alternatively be a glycidyl ether of a polyhydric phenol, for example an epoxy novolak resin, or an aliphatic or cycloaliphatic di- or poly-glycidyl ether. Examples of epoxy resins containing glycidyl ester groups are homopolymers or copolymers of a glycidyl ester of an ethylenically unsaturated carboxylic acid such as glycidyl methacrylate or glycidyl acrylate, or the diglycidyl ester of dimerised fatty acid.
In most cases the curing agent preferably contains more than two, for example at least three, heterocyclic secondary amine groups, although mixtures of a curing agent with at least three such groups and a curing agent with at least two but less than three such groups can be used. It is strongly preferred that either the epoxy resin contains an average of more than two epoxy groups per molecule or the curing agent contains an average of more than two heterocyclic secondary amine groups per molecule, for example at least 2.5 or 3 heterocyclic secondary amine groups per molecule. The heterocyclic secondary amine groups can for example be part of a heterocycle containing 3 to 12 atoms in the ring, for example a saturated heterocycle such as a piperidine, piperazine, pyrrolidine, azetidine, aziridine, imidazolidine, oxazolidine, thiazolidine or homopiperazine (1,4-diazacycloheptane) ring, an unsaturated heterocycle such as an imidazoline ring or even an aromatic ring having a secondary amine group such as pyrrole or imidazole. For rings containing two hetero-N-atoms such as piperazine or imidazolidine, it is usually preferred that only one N atom in an individual ring is present as a secondary amine group; the ring can be attached to the remainder of the curing agent molecule through the other N atom, for example the other N atom of a piperazine ring.
Thus, according to another aspect of the invention a coating, sealant or adhesive composition curable at ambient temperatures of 40xc2x0 C. or below comprises an epoxy resin and an amine-functional curing agent and is characterised in that the curing agent comprises a material containing at least three heterocyclic secondary amine groups.
The curing agent can for example be the reaction product of a primary amino-substituted heterocyclic secondary amine with a compound containing two or more, preferably at least three, groups which are reactive with primary amine groups but substantially unreactive with heterocyclic secondary amine groups. The primary amino-substituted heterocyclic secondary amine can for example be N-(2-aminoethyl)piperazine, 2-(2-aminoethyl)imidazoline, N-(3-aminopropyl)piperazine, 4-(aminomethyl)piperidine, 2-(aminomethyl)piperidine, 3-(aminomethyl)piperidine or 3-aminopyrrolidine, or a substituted derivative of any of the above containing for example one or more alkyl or alkoxy substituents. The groups which are reactive with primary amine groups but not with heterocyclic secondary amine groups can for example be beta-dicarbonyl groups such as acetoacetate groups and similar beta-ketoester groups or beta-diketone groups, other aldehyde or ketone groups, for example the aldehyde groups of glutaraldehyde, terminal urea groups xe2x80x94NHCONH2 or imide- forming groups such as cyclic anhydrides or half-esters of vic-dicarboxylic acid groups capable of forming cyclic imides. Acid groups, particularly carboxylic acid groups, and their lower alkyl esters will also react preferentially with primary amine groups rather is than heterocyclic secondary amine groups. The curing agent can alternatively be formed by reaction of a polyfunctional reagent with pyridine substituted by a reactive functional group, followed by hydrogenation of the pyridine ring to generate secondary amine groups.
A di- or poly-acetoacetate ester, for example, can be reacted with N-aminoethylpiperazine or another primary amino-substituted heterocyclic secondary amine to bond the heterocylic secondary amine to the acetoacetate ester through an imine or enamine linkage. The reaction is shown below for trimethylolpropane tris(acetoacetate). 
trimethylolpropane tris(2-piperazinoethyl)aminocrotonate (may exist in ketimine or enamine form)
Trimethylolpropane tris(acetoacetate) can be prepared from trimethylolpropane and a lower alkyl acetoacetate such as t-butyl acetoacetate by heating to transesterify, with removal of the volatile alcohol such as t-butanol by distillation. Poly(acetoacetate)esters can similarly be formed from other polyols such as pentaerythritol, 1,6-hexanediol, trimethylolethane or sorbitol or hydroxy-functional polymers such as acrylic polymers having pendant hydroxyl groups, for example homo- and co-polymers of 2-hydroxyethyl acrylate or methacrylate and polyesters, including hyperbranched or dendritic polymers having surface hydroxyl groups and 2,4,6-tris(hydroxymethyl)phenol and other compounds and oligomers formed by novolak condensation of an optionally substituted phenol and formaldehyde.
Glutaraldehyde will react with a primary amine to form one imine linkage followed by aldol condensation of the other aldehyde group to form a hydroxy-substituted polymer chain, provided that the reaction is carried out without having a significant molar excess of glutaraldehyde over amine at any stage. The hydroxy-substituted polymer chain may undergo further dehydration to form an unsaturated polymer chain. If glutaraldehyde is reacted with aminoethylpiperazine, a polymer is formed having pendant imine groups of the formula: 
A di- or poly-urea can be reacted with N-aminoethyl-piperazine or other primary amino-substituted heterocyclic secondary amine to replace the terminal xe2x80x94NH2 of the polyurea with elimination of ammonia. The reaction is shown below for hexamethylenediurea. 
Di- and poly-ureas such as hexamethylenediurea can be prepared by heating a di- or poly-amine such as hexamethylenediamine with an excess of urea. The amino groups of the di- or poly-amine are preferably primary amino groups, although secondary amino groups will also react. The di- or poly-amine can for example be ethylene diamine, diethylene triamine or tris(2-aminoethyl)amine.
Examples of compounds and polymers containing carboxylic acid or lower alkyl ester groups which will react preferentially with primary amine groups are polyesters having terminal acid or ester groups, acrylic acid polymers or methyl or ethyl acrylate polymers, Michael-type adducts of an unsaturated carboxylic acid ester such as dimethyl maleate with a Michael donor such as a thiol or a malonate or acetoacetate ester, ester-substituted lactone reaction products of an epoxy resin with dimethyl malonate, or dimer fatty acids or acid-terminated low melting polyamides derived from them. The lower alkyl ester groups are ester groups of a lower boiling alcohol such as an alcohol having 1 to 4 carbon atoms, especially methyl or ethyl ester groups. The acid or ester groups react with primary amine groups to form amide linkages.
Although carboxylic acid groups react preferentially with primary amine groups, they will react with heterocyclic secondary amine groups under more forcing conditions, and a compound or polymer containing at least two heterocyclic secondary amine groups can be formed by reaction of a compound or polymer, for example a copolymer of acrylic acid with one or more acrylate or methacrylate ester monomers, having at least two carboxylic acid groups with piperazine. The piperazine reacts predominantly to form a monoamide of each piperazine unit.
The curing agent can alternatively be the reaction product of a hydroxy-substituted heterocyclic secondary amine such as N-(2-hydroxyethyl)piperazine with a compound or polymer containing at least two groups reactive with hydroxy groups under conditions in which the heterocyclic secondary amine does not react. For example, hydroxyethyl piperazine can be reacted with a polyisocyanate to form a urethane having at least two piperazine secondary amine groups if the amine group is first deactivated by salt formation with a strong acid. The amine groups need to be reactivated by removal of the acid before the reaction product is used as an epoxy curing agent.
When reacting a primary amino-substituted heterocyclic secondary amine such as aminoethyl piperazine with a multifunctional cyclic anhydride to form imide linkages, care should be taken to avoid gelation. The multi-functional cyclic anhydride can for example be ring-opened with an alcohol to form a half-ester, which will itself form imide linkages with aminoethyl piperazine on heating. Examples of multi-functional cyclic anhydrides are maleic anhydride polymers, for example copolymers with styrene or with an alpha-olefin such as 1-octene or with one or more acrylate or methacrylate ester monomers. Itaconic anhydride polymers can be formed from itaconic acid as described in GB-A-2137637.
The curing agent can be a polymer or oligomer containing secondary amine-containing heterocyclic rings either pendant from a polymer chain or linked in a polymer chain. Examples of heterocyclic rings pendent from a polymer chain are described above. Further examples can be derived by hydrogenation of a polymer containing pyridine groups such as poly(vinyl pyridine). Another example is a siloxane polymer containing pyridine groups which can be formed by reaction of vinyl pyridine with a polymer containing Si-H groups, for example a poly (methyl hydrogen siloxane), in the presence of a hydrosilylation catalyst such as a platinum compound. Compounds and polymers containing 3 or more pyridine groups can be formed by reaction of vinyl pyridine with a polythiol such as pentaerythritol tetra (3-mercaptopropionate). 4-Aminopyridine can be reacted with di- or poly-acid to form a di- or poly-amide containing pyridine groups or with a diacrylate to form a Michael-type adduct containing pyridine groups or with a diepoxide to form an adduct containing pyridine groups. Any of these materials can be hydrogenated to form a poly(piperidine), as can terpyridine. Analogous polymers and oligmers containing 5-membered secondary amine heterocycles can be formed from 2- or 3- vinyl pyrrole or vinyl imidazole followed by hydrogenation.
Polymers and oligomers containing pyrrolidine groups can be produced by lithium aluminium hydride reduction of maleimide or succinimide groups. An addition copolymer of maleimide (whether formed by polymerisation of maleimide or by polymerisation of maleic anhydride followed by reaction with ammonia) can be reduced to a polymer containing pyrrolidine groups linked in a polymer chain. A maleinised polymer or maleinised polyene, for example polybutadiene maleinised by reaction with maleic anhydride, or an itaconic anhydride polymer, can be reacted with ammonia to form maleimide groups which can be reduced to pyrrolidine groups. The polyene can for example be a cyclic polyene such as cyclododecatriene. A cyclic oligomer containing pyrrolidine groups can alternatively be prepared by hydrogenation of a macrocycle containing pyrrole groups, for example an acetone pyrrole reaction product. 
which can be hydrogenated to form a tetrapyrrolidine useful as a curing agent.
A polymer containing piperidine and/or pyrrolidine groups in the polymer chain can be produced by the free is radical initiated cyclopolymerisation of diallylamine. 
Epoxy resins cure particularly rapidly with the cyclopolydiallylamine polymerisation product. This can be valuable for a coating or sealant required to cure at particularly low temperature. The cyclopolydiallylamine can be partially reacted with a monofunctional reagent such as an epoxide or acrylate prior to the curing reaction with an epoxy resin to reduce its reactivity.
Heterocyclic amines can alternatively be formed by the reaction of ammonia with 1,2,5,6-diepoxides. The diepoxides can for example be derived from a sugar-derived polyalcohol such as D-mannitol, as described by L. Poitout et al in Tet. Lett. 35 3293 (1994). Heterocyclic secondary amines are formed as shown below: 
where P=protecting group.
Reaction of the diepoxide with ammonia leads to ring opening of one of the epoxides followed by spontaneous intramolecular ring closure to give the piperidine and/or the azepine. The central hydroxyl groups may be used, for example by prior reaction with isocyanate groups, to link the piperidine and/or azepine to an appropriate core unit to give multifunctionality. The diepoxide can is alternatively be derived from a polydiene, for example from 1,2- or 1,4-polybutadiene.
A curing agent containing oxazolidine groups can be prepared from a polyepoxide by reaction with ammonia followed by reaction with formaldehyde as shown below for one of the epoxide groups: 
The curing agent can comprise a simple di(secondary amine heterocycle) such as 1,3-bis(4-piperidino)propane or the 1:2 molar adduct of urea and N-(2-aminoethyl)piperazine. 
In general, these curing agents have the disadvantage that they are only di-functional and are often solids which are difficult to mix into curing compositions. They are preferably not used as the only amine curing agent for epoxy resins having an average epoxide functionality of 2 or less. They can advantageously be used mixed with other heterocyclic secondary amine curing agents having a functionality greater than 2, particularly liquid oligomers and polymers. The di(secondary amine heterocycle) curing agents can also be used as curing agents for epoxy resins of higher functionality such as glycidyl acrylate or methacrylate polymers or epoxy novolak resins. The di(secondary amine heterocycle) compounds can alternatively be reacted with a polymer to form a polymeric curing agent tipped with heterocyclic secondary amine groups, for example 1,3-bis(4-piperdino)propane can be reacted with a polyepoxide at a ratio of at least two piperidine groups per epoxy group to form a curing agent.
When the curing agent and epoxy resin are mixed to form a coating, sealant or adhesive composition the curing agent and epoxy resin are usually used in amounts s such that the composition contains at least 0.5 heterocyclic secondary amine groups per epoxide group, although lower amounts of amine can be used if the epoxy resin contains many epoxy groups, for example a glycidyl acrylate or methacrylate polymer. The composition preferably contains at least 0.6 up to 1.5 or 2.0 heterocyclic secondary amine groups per epoxide group, although higher amounts of the curing agent can be used if it contains many heterocyclic secondary amine groups.
The curing agent according to the invention can comprise a blend of different heterocyclic secondary amines. Normally the total curing agent present has an average of at least two heterocyclic secondary amine groups per molecule. When the epoxy resin has high functionality, for example a glycidyl acrylate or methacrylate polymer, the curing agent can be used in conjunction with a heterocyclic secondary amine having only one secondary amine group, and in these circumstances the total curing agent may have an average of less than two heterocyclic secondary amine groups per molecule.
A curing agent containing heterocyclic secondary amine groups can also be used in conjunction with another curing agent for the epoxy resin such as a primary amine. Examples of such primary amines are 1,3-bis(aminomethyl)cyclohexane, m-xylylene diamine, isophorone diamine, bis(4-aminophenyl)methane and bis(4-aminocyclohexyl)methane or even a simple primary monoamine such as n-hexylamine, n-octylamine or benzylamine. Preferably, the other curing agent does not constitute more than 50 percent of the total amine nitrogen of the curing agent.
The compositions of the invention are used as coatings, adhesives or sealants (including potting compounds used for sealing electrical and electronic components) and are particularly suitable for curing at ambient temperature, for example below 40xc2x0 C. and particularly below 25xc2x0 C. and including cold climates where the ambient temperature is below 10xc2x0 C. or even below 0xc2x0 C. down to xe2x88x9210xc2x0 C. or xe2x88x9220xc2x0 C. The compositions of the invention cure and harden more rapidly than epoxy resin compositions based on conventional primary amine curing agents, for example they cure 2 to 10 times faster at 10-20xc2x0 C. than similar compositions based on bis(aminomethyl)cyclohexane as curing agent. The compositions of the invention can be prepared at sprayable viscosity at high solids (250 g/L or less of volatile organic material). Despite the tendency of high solids coatings to cure more slowly, coatings according to the invention can cure more rapidly than conventional amine-cured epoxy coatings of higher solvent content. The reaction rate of compositions according to the invention is about 8 times faster than that of epoxy resins cured by a hydrogenated aralkyl primary diamine, as measured by ASTM E698-79.
The compositions of the invention can for example be used as the binder of anticorrosive primer coating compositions applied to steel used in shipbuilding or structural steel used in buildings, bridges or oil rigs. Such compositions generally have to be cured at ambient temperature. The epoxy resin and the curing agent are generally stored separately and are mixed shortly before application together as a coating to the substrate, for example 10 minutes to 2 hours before application, or they can be mixed at application in a twin-feed spray and applied together. The coatings generally harden sufficiently rapidly at 10xc2x0 C. (often at 0xc2x0 C.) that a 200 xcexcm coating film can be trodden on 24 hours after application, usually 8 hours after application, without gouging of the film.
The compositions may optionally contain compounds known to accelerate the epoxy-amine reaction if even faster cure is desired. Many additives tested as cure accelerators, for example Lewis acids, are described in the list compiled by Inoue in the Proceedings of the 21st Japan Congress on Materials Research, page 251 (1979) Hydrogen bond donor materials such as alcohols, phenols and acids accelerate the cure. A common example is 2,4,6-tris(dimethylaminomethyl)phenol. Metal salt catalysts such as calcium nitrate can alternatively be used.
The coatings may contain an anticorrosive pigment such as metallic zinc, zinc phosphate, wollastonite or a chromate, molybdate or phosphonate, a barrier pigment such as micaceous iron oxide, glass flake, aluminium flake or mica and/or a filler pigment such as iron oxide, barytes, talc, calcium carbonate or titanium dioxide. The coatings may also contain other coating additives such as a wetting agent, a thixotrope, a reactive diluent, for example a monoepoxide, a flow control agent, solvent or diluent. Sealant compositions generally contain a filler and/or pigment such as those listed above, and sealant or adhesive compositions may contain additives such as those listed for coating compositions. The pigments and other additives can be mixed and stored with the epoxy resin or with the curing agent or with both.
The invention is illustrated by the following Examples, in which percentages are by weight unless otherwise indicated: